12 



K. MITSUKUEI ; 



capitulate here briefly what has been given in the previous contribu- 

 tions, so far as is necessary to make what follows comprehensible. 

 The facts are mainly contained in Conlvihution II \ AY hen seg- 

 mentation has o-one on for some 



Woodcut IV. 



time, there is established in the 

 blastoderm two layers : ((() the 

 superficial epiblast composed of 

 columnar cells, and (J)) the lower 

 layer consisting' of irreo-ular 

 stellate cells and probably not 

 forming a complete sheet (Wood- 

 cut IV. A). This separation into 

 the two layers takes place in all 

 parts of the blastoderm with the 

 exception of a small area at the 

 posterior end of the embryonic 

 shield. Here not only is there 

 no differentiation of layers, but a 

 thick knob (iV. Kn.) consisting 

 of a netw^ork of cells is produced 



Diagrcams showing the course of gastrulation \^y ^|-^g ^CCeSftion of Cclls fiv^m the 

 in Chelonia. 



subjacent bed of yolk. This is 

 the Primitive Knob or Primitive Plate. The mass can not be said to 

 belong to either of the two layers above named. According to my 

 observations, the two layers pass gradually into the primitive knob, 

 and nowhere can I discover such a sharp line of demarkation as is 

 described by Will (Woodcut IL, A). AVitliin the area of this knob, 

 an invagination cavity is produced, which at first goes straight down- 

 wards, but very soon takes a forward horizontal course (Woodcut IX., 

 B, C). This I take to be the Archenteron and its dorsal opening to be 



